Abstract Details

Electrophysiological Characteristics of Voluntary Orthostatic Leg Tremor

Orthostatic tremor (OT) is a rare disorder characterised by symptomatic unsteadiness and tremor when standing unsupported. The frequency of EMG bursts and frequency/magnitude of EMG-EMG coherence calculated between lower limb muscles supports the diagnosis.

High frequency (13-18 Hz) EMG bursts were a defining feature in the original description of primary OT. More recently, 'slow' OT with EMG bursts at frequencies of 3-12 Hz has been described (Leu-Semenescu et al, 2007). At this frequency, it is possible to imitate orthostatic tremor. We therefore recorded EMG from healthy controls whilst imitating OT with the purpose of defining electrophysiological features of voluntary OT and refining diagnostic criteria for pathological 'slow' OT. 

Experiments in healthy controls were approved by the Newcastle University Research Ethics Committee. Thirty-six participants were involved in the study (18 males; age-range 20-85). Surface EMG was recorded simultaneously from tibialis anterior, medial gastrocnemius, vastus lateralis and biceps femoris bilaterally. EMG was amplified (5K) and filtered (3 Hz-2 KHz) using an 8-channel D360 amplifier (Digitmer), and digitized at 5 KHz (CED 1401). Participants were asked to mimic OT by shaking their legs synchronously and as rapidly as possible. In some participants, time to fatigue and cessation of movements was recorded (n=18). Data from healthy controls was compared with anonymized data recorded from a sample of patients with primary OT (n=6). EMG data was analysed using Spike 2 (CED) and MATLAB software. 

EMG burst duration in voluntary OT (50-90ms) was significantly different from mean EMG burst duration (<50ms) in patients with primary OT (t-test; p<0.05). There was a significant difference between peak tremor frequency and magnitude of EMG-EMG coherence in healthy controls and patients with primary OT (t-test; p<0.05). There were significant negative correlations between age and task duration (R2=0.053) and burst frequency and task duration (R2= 0.058). 

By incorporating additional electrophysiological parameters to diagnostic criteria for OT, this disorder, particularly the newer entity of 'slow' OT, can be diagnosed with greater confidence. 

TitleForenamesSurnameInstitutionLead AuthorPresenter
MissAlexis CollinsNewcastle University
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